The real trick is if the reeds are rusty (seriously rusty, a slight patina may be normal on many old reeds). Then you WILL need new reeds. Also you may find then that all the metal bits may be corroded to the extent that you have an obect of art, not a playable musical instrument. Moisture and heat are the enemies of accordions.

For the totally technically ignorant about the insides, you should have someone who really knows guide you a bit. If the seller permits it, you can easily remove the bellows to look at the reed blocks. There's a few pins that can be pulled out (using pliers with care) to unfix the bellows - if the bellows leak, or have holes in, this instrument is not for beginners. The cost of a new replacement bellows will be more that what you pay for the instrument. If the basic instrument is good, it will be worth it, but if the bellows have died, then consider why, and think what else may have died inside.

Once you have the bellows out of the way, you can see the condition of the reed block, which are easily removable - there are little sliding tabs of metal - DON'T FORCE ANYTHING! The little flaps of leather/plastic are the valves - if dead, they are relatively cheap to get replaced. Piano accordions have each note pitch with two reeds - one for each notes for each direction, you need to look inside the reed block to see the other one.

Good hunting - you may become obsessed like me... and then even if you are not capable of actually doing the repair work yourself, at least you will be able to establish whether an instrument is is worth it. Get a reputable repairer to "give you the tour" inside an instrument in good condition, and you will start to know what to look out for.

There are terrible old boxes out there, but there are many good ones still left that have been kept reasonably well - many were played a little then packed away for forty years. The storage itself may have caused some problems, but many of those - provided that insects, heat and moisture were excluded, little serious damage should have occurred. If not stored standing on its little rubber feet - its base - or the case stored with it lying on its feet - then the valves may have been distorted by gravity, and need to be replaced. Your repairere will easily demonstrate why this occurs.

Have fun!

Robin